A variety of methods for transmitting high-quality and high-volume data using limited radio resources has been appeared in current wireless communication systems. An orthogonal frequency division multiplexing (OFDM) employs a plurality of subcarriers having orthogonality.
A codeword refers to a bit stream which is generated by subjecting information bits comprised of ‘0’ and ‘1’ to a specific rule so as to improve detection performance in the presence of noise.
In wireless communication system, there are two detection methods to reproduce information bits from a codeword. In coherent detection, a pilot symbol is used to estimate a channel. In non-coherent detection, orthogonal codes is used to detect a codeword.
In coherent diction, a receiver estimates a channel using a pilot symbol. The pilot symbol is data that is known to both a base station and a user equipment. A received signal y can be represented as shown
MathFigure 1y=hx+v  [Math.1]
where x denotes a transmit signal, h a channel, and v an Additive White Gaussian Noise (AWGN).
Since the pilot symbol has been already known, a channel for the pilot symbol can be estimated. The received signal y can be reproduced as shown
MathFigure 2
                              y                      h            ^                          =                                            hx              +              v                                      h              ^                                ≃                      x            +                          v              _                                                          [                  Math          .                                          ⁢          2                ]            
where
ĥ
denotes estimated channel for the pilot symbol. As a result, it is required to accurately estimate a channel to reduce detection error.
Since data is reproduced using a channel for the pilot symbol, a channel for the data has to be same with the channel for the pilot symbol in order to achieve ideal detection. But the channel for the pilot symbol is different with the channel for the data in time domain and/or frequency domain. In other word, detection performance is better as the pilot symbol is closer to the data and the number of pilot symbols is larger. But as the number of pilot symbols increase, data rate may be constrained.
In non-coherent detection, a codeword is created by orthogonal codes. The codeword is detected by a correlator. The detected codeword is demapped to obtain information bits. Although a pilot symbol is not necessary, complexity of the correlator is taken into consideration.
A codeword using channel coding shows poor detection performance when the length of the codeword is not enough long. In addition, there occurs performance degradation under time-varying channel due to channel estimation error.
A codeword using the orthogonal codes shows relatively good detection performance even when the length of the codeword is shorter than that of the codeword using the channel coding. Detection performance does not depend on error of channel estimation. But the complexity of a correlator increases as the length of the codeword is increased. Therefore, a method is sought for efficiently generating a codeword without requiring the channel estimation. Moreover a method is sought for reducing complexity of the correlator in a receiver.